1. Field of the Invention
This invention relates to a method for extraction of pollutants from underground water using vertical circulating wells and cyclodextrins.
2. Art Relating to the Invention
Underground water is present in a number of sub-surface formations such as aquifers, sediment, and vadose zones. Oftentimes, these sub-surface formations are polluted with organic compounds, inorganic compounds, and combinations thereof. These pollutants are generally toxic to humans and animals, and the pollution of sub-surface water is a major environmental problem.
One standard method for extracting pollutants from sub-surface water is to use vertical circulating wells. Such wells are drilled into the polluted sub-surface and water is pumped into the strata in a vertical flow, perpendicular to the strata, at one level and removed at a lower level. The removed water is then treated to remove the pollutant and pumped back into the well. This flushing technique is continued until the concentration of pollutants in the water reaches an acceptable level.
Cyclodextrins are a non-toxic, biodegradable material which has been used in the past to extract organic compounds from soil. For example, see U.S. Pat. No. 4,255,249 which relates to the extraction of oil from oil sand; and U.S. Pat. No. 5,425,881 which relates to the extraction of hydrocarbon pollutants from contaminated soil. Both the ""249 and ""881 patents require mixing cyclodextrin or an aqueous solution of cyclodextrin with the soil and then extracting the complex of cyclodextrin and hydrocarbon from the soil.
Cyclodextrins are made from starch through the use of an enzyme, cyclodextrin glycosyltransferase (CGT). Typically, the starch is first gelatinized and then liquefied to a DE of 1 to 5 with either an acid or enzyme. The CGT enzyme forms alpha-, beta- and gamma-cyclodextrins, both branched and unbranched.
One of the unique features of cyclodextrins is their ability to form complexes with other compounds. The cyclodextrin has a torus structure where the interior is hydrophobic and the exterior is hydrophilic. Oftentimes, the exterior is modified to increase the hydrophilic nature of the cyclodextrin.
The use of other compounds such as cosolvents and surfactants have been suggested for use in vertical circulating wells. The disadvantage to cosolvents and surfactants is that they themselves are oftentimes pollutants which must be removed from the sub-surface water.
There is a need for a better extraction process of pollutants from ground water.
Applicants have now discovered that a wide range of pollutants can be removed from sub-surface water by using cyclodextrins in the water pumped into a vertical circulating well. The cyclodextrin is allowed to travel with the water as it sweeps from its inlet point to its outlet or recovery point in the sub-surface formation without the need for agitation. This is especially novel because cyclodextrins are generally not used to complex with another compound without mixing or agitation.
Applicants have also learned that the complex of cyclodextrin with the pollutant can be broken by an air stripping process. Specifically, a countercurrent flow between a downward flow of water containing the complex and upward moving air causes the pollutant to separate from the cyclodextrin. This is especially novel because conventionally these complexes are separated into their component parts by heat, e.g. boiling an aqueous solution of the complexes and then separating the cyclodextrin from the other components.
Broadly, the process of the present invention entails the extraction of pollutants from sub-surface water comprising:
a) introducing an aqueous cyclodextrin solution into a sub-surface area containing water with pollutants;
b) allowing said aqueous cyclodextrin solution to flow downward through said sub-surface area to form complexes with said pollutants; and
c) recovering water with said complexes from an area below where said aqueous cyclodextrin solution was introduced into said sub-surface area.
Preferably, the process of the present invention further includes separating said cyclodextrin from said pollutants and recycling said cyclodextrins to use the cyclodextrins again in step (a). More preferably, this separation is conducted by air stripping the pollutant from the cyclodextrin.
Preferably, a vertical circulation well is employed wherein the aqueous solution of cyclodextrin is introduced with a diffuser ring partway down the well and the water with the complexes is recovered at the bottom of the well. As will be appreciated, such a system employs a single well to both introduce and recover the water through a vertical flow pattern that sweeps the treated sub-surface area.
Liquid pumps are suitably used to move the fluids throughout the process. Gravity causes the aqueous cyclodextrin solution to move downward, perpendicular to the strata, in a vertical flow pattern. The contact between the downward flowing cyclodextrin and the pollutant in the water causes the complexation to take place.
The use of cyclodextrins to extract pollutants from sub-surface water has a number of advantages. It has been found that the cyclodextrins have an insignificant reactivity with porous underground formations; that the cyclodextrins are insensitive to pH, ionic strength, presence of salt and other sub-surface environmental factors; that the cyclodextrins have a low potential to mobilizing pollutants away from the recovery area; that cyclodextrins are reusable, thus, keeping cost down; and that cyclodecxtrins are non-toxic and biodegradable such that recovery of the cyclodextrins from the sub-surface area is not necessary.